This is a proposal for continuing long-term study of kinetics of permeability of certain biological membranes and their modifications by certain peptide hormones, particularly insulin. A unitary hypothesis has been developed to account for actions of peptide hormones, particularly insulin. The hypothesis involves association between hormone and outer elements of susceptible cell membrane, re-organization of the membrane as a means of transmitting information to the interior, with modulation of permeability and of membrane-linked transport and synthesizing systems. Tests of the hypothesis will be made on isolated adipose cells and on membrane fragments from skeletal muscle.